A Missing Chapter From Earth's History May Have Been Crushed by Glaciers

The Great Unconformity isn't just a nickname for your teenage years.

A huge section of Earth’s fossil history is missing. Scientists know where it should be, but it’s not there. And now they think they know where it went.

All over the planet, there are huge gaps in the geological record right before the Cambrian period, when life exploded. In the Grand Canyon, for instance, the Paleoproterozoic Vishnu Schist sits directly beneath the Cambrian Tapeats Sandstone, even though hundreds of millions of years separated those two time periods — and the rock from the time in-between is nowhere to be found. This strange phenomenon is called the Great Unconformity, and for over a century it’s been one of the biggest mysteries in geology. But new research published in the Proceedings of the National Academy of Sciences proposes a tidy explanation for the disappearance of as much as 1.2 billion years worth of rock — 3 to 5 vertical kilometers — from all over the globe.

In a paper published December 31, an international team of researchers explains that a global glacial period known as “Snowball Earth” about 700 million years ago could explain the Great Unconformity. During this hypothesized period, the scientists argue, glaciers covering the surface of the Earth eroded rock that had accumulated for millions of years. Much like the more recent glaciers that carved out the epic landscapes that still exist on Earth today, these older glaciers, with their immense pressure and friction, slowly carried sedimentary rock away from its origin. Eventually, the glaciers dumped their loads in the ancient oceans, where the rocks were folded back into the Earth’s mantle via seafloor subduction zones. Once the missing rock returned to magma, it would one day be expelled through the Earth’s crust.

This image shows how rock from two very distant time periods sit directly on top of each other in the Needle Mountains of Colorado.

This explanation would neatly explain how the missing rock of the Great Unconformity had disappeared from where scientists would expect to find it, as well as why it hasn’t been found anywhere else on the planet. Essentially, they argue, it was re-melted in Earth’s furnace and redistributed around the world. It’s a very tidy, almost elegant explanation. As such, the researchers expect to receive pushback from their peers.

“I think, though, we have extraordinary evidence to support that extraordinary claim,” the study’s first author C. Brenhin Keller, Ph.D., a postdoctoral fellow at the Berkeley Geochronology Center in California, told National Geographic.

This evidence is found in ancient zircons, minerals whose chemical compositions preserve clues about the conditions that were occurring on Earth when they crystallized out of cooling magma. They’re like mineral metadata from the time period. If the team’s hypothesis is true, then the world’s zircon should bear the chemical signature of a huge quantity of rock getting recycled into the Earth’s mantle all at once. So they analyzed samples of magma and compared it to a theoretical model of how this scenario should appear in the zircon record. And the model explained the evidence just as the team found it.

“We are talking about an absolutely huge amount of crust being eroded,” Keller told the LA Times. “In which case, we should have noticed it missing — and we have.”

The signature comes in the form of oxygen and hafnium isotopes, which Keller and his team found in such large quantities that it could only be explained by a massive loss of one-fifth of the geological record from the face of the Earth.

This paper is just one more step in the process of understanding why there are huge gaps in the fossil record, but if other researchers can confirm the findings, it puts a nice little bow on a mystery that has vexed geologists since the 1800s.